How to unwrap model so its perfectly proportioned?
1:  Mark one vertical edge as Seam. In my example I added a seam on a very visible edge but ideally this seam should be placed somewhere less obvious.

2: Select all of the mesh, open the UV mapping menu and choose the Unwrap option.

3: Open the  UV/image editor window. The unwrap should look similar to the image below. This now needs to be scaled up.



4: UV Editor window, Make sure the Cursor is at the X=0 Y=0 position of the UV space. The lower left hand corner of the UV unwrap should also be at this position.

5: Change the Scaling Pivot point to be the 2D Cursor.

6: Scale up the UV’s until the highest edges are approximately at the same level as the top of the UV space.  



7: To check seams, scale etc  Add a test texture/image in the UV editor  and see how it looks on the mesh.



8: With the 2D cursor still at the lower left hand corner of the UV space scale up or down until the test texture looks correct at the seam.